The principle of energy dissipation in rock impacted by impulse loading was investigated. Associated with damage mechanics, a new method was proposed to calculate the cumulative damage of rock under a repeated loading at the damage stage, which was studied from the point of the action density of energy. The concept of abstract fractal characteristic of damage parameter was put forward based on the intrinsic physical meaning of damage,which was applied to the fragmentation phase of rock. The relationship between fractal damage and the number of fragments was set up. According to the principles of energy dissipation and dynamic fracture of rock experiencing impact load, the damage evolution equation in the fragmentation phase of rock was presented. The results of experiment show that the two methods coincide well with each other.